There is an undeniable link between the frequency of Demodex occurrence and the age of the host. As age increases, the number of potential contacts with infected individuals also rises. For example, the following observations of the occurrence of folliculorum and D. brevis (number of individuals tested N=435; age from 3 to 96): 13% of individuals aged 3-15 years; 34% of individuals aged 19-25 years; 69% of individuals aged 31-50 years; 87% of individuals aged 51-70 years, and 95% of individuals aged 71-96 years. Other studies show, for example, infestations with Demodex among first-year students at 43% and third-year students at 67% (N=2200). Similar research is abundant in the literature and indicates a consistent trend of increasing infestation with age.
The results of studies attempting to demonstrate the influence of host gender on the degree of Demodex infestation are conflicting. While one group of authors describes a higher intensity of infection in women than in men, attributing the cause to the use of creams and powders and the influence of sex hormones, another group points to a higher percentage of infections in men, without clear indications of the cause. A third group of researchers does not observe a connection between gender and Demodex occurrence. The random nature of Demodex spread in the human population would suggest agreeing with this third group of researchers.
Demodex mites are common throughout the human population, regardless of geographical location. They have been found among Europeans, Asians, Americans, as well as Eskimos, Nigerians, and Aboriginals. Some reports indicate a higher frequency of infections observed among residents living in humid climates (68%) than among those living in dry climates (25%). However, many studies have been conducted on too small samples and with different methodologies. Comparing such diverse studies presents difficulties in interpreting the results, often leading to false or unjustified conclusions.
There are professions whose workers are more exposed to the possibility of Demodex infection than others. These include, for example, employees of social care homes and workers in medical professions (doctors, nurses, physiotherapists). These individuals have frequent and close contact with elderly people.
Previous findings from various researchers suggest that people with dark hair or bald individuals are more frequently infected. However, these observations are based on limited material and require verification.
There are also reports indicating that a higher level of infection concerns individuals with oily skin and seborrheic tendencies. For example, results show that Demodex infection was noted in 90% of those examined with seborrhea, 57% with normal skin, and 52% with dry skin. Other studies reported Demodex infestation rates of 47% in individuals with oily skin, 27% with dry skin, and 34% with mixed skin. It was also found that an increase in the pH of the stratum corneum of the epidermis may adversely affect the homeostasis of the barrier permeability, integrity, and cohesion of the stratum corneum, thereby providing suitable conditions for the development of Demodex.
A higher level of infestation has been noted in individuals with a dark complexion. However, these observations, like those described above, are based on limited material and require verification.
Several results indicate that individuals with dermatological diseases such as rosacea and acne are more susceptible to Demodex infestation than healthy individuals. It has also been observed that Demodex occurs more frequently in patients with lowered immunity (HIV, leukemia, patients undergoing antibiotic therapy, hormonal therapy, immunosuppressive drugs, chemotherapy, hemodialysis, peritoneal dialysis, AIDS patients, leukemia, Behçet’s disease, recurrent styes, eyelid inflammation, skin cancer, urinary tract cancer, gastrointestinal cancer, breast cancer, lung cancer, non-Hodgkin lymphoma, chronic lymphocytic leukemia, and myeloid leukemia, basal cell carcinoma of the eyelids, plasmacytoma, malignant granuloma, lymphatic sarcoma). A higher frequency of demodicosis was also shown among malnourished children. Conflicting results were obtained for individuals with diabetes, end-stage chronic kidney disease, and rheumatoid arthritis.